7 Turbine rotors and couplings
7.1 Types of rotor construction
Four different types of rotor construction have been used on large turbine-generator units on the CEGB system:
- The monobloc or integral rotor, in which the wheels and shaft are formed from a single-piece forging (Fig 1.104 (a)).
- The built-up or shrink-on disc rotor, consisting of a forged steel shaft onto which separate forged steel discs are shrunk and keyed (Fig 1.104 (b)).
- Drum type rotor manufactured from solid or hollow forgings (Fig 1.104 (c) and 1.104 (d)).
- The welded disc rotor (Fig 1.104 (e)). These rotors are not too common in the UK, where they have been applied to LP turbine rotors. Overseas applications have included HP and IP turbine rotors.
For various reasons, monobloc forgings are preferred but where the size has exceeded the forging capability, the built-up disc construction has been used. The current 660 MW UK designs all have monobloc rotor forgings.
Built-up rotors required very careful attention to shrink fit and location geometries to avoid problems in running and with fatigue cracking. While the discs may have facilitated non-destructive testing (NDT), the NDT capability on monobloc rotors has been developed to meet all the needs. With the monobloc method of construction, the LP rotors are more rigid, resulting in better dynamic behaviour. 660 MW plant employs rotors of this construction almost exclusively, and experience has been good.
It was the practice to provide test material from a bore-hole down the forging axis but, as confidence in forging practice and material properties has increased, the central bore has been omitted on some current designs. The benefit of reduced stress levels gives enhanced confidence in the longer-term fatigue integrity of the rotor.
Welded rotor construction has the advantage of smaller forging components at the expense of high integrity welding. The welded design, having been adopted by countries lacking an intrinsic large forging capability, has been successfully applied by them to HP, IP and LP rotors. There are a limited number of welded LP rotors in service in the UK.
High temperature drum-type rotors, manufactured from hollow cylinders bolted to stub shafts, have been prone to differential creep and have been replaced by monobloc drum rotors in current designs. Where constraints on last-stage blading design dictate, double-flow cylinders replace the single-flow design as used in the HP turbine. Double-flow IP and LP turbines are standard for 660 MW designs. With the single-flow HP turbine, the axial thrust has to be balanced to some extent by a 'balance piston' to reduce thrust bearing loads: for reaction turbines particularly (with a high pressure drop across the moving blades) the balance piston is of substantial proportions, as shown on Fig 1.105.
In contrast, an HP turbine rotor with impulse blading is illustrated on Fig 1.106. This arrangement is characterised by the reduced number of blading stages and larger blading pitch diameter compared with the reaction design. In addition, a very much reduced balance piston is necessary, as the axial thrusts are lower.